Digital image sensing based upon solid state technology is well known, the two most common types of image sensors currently being charge coupled devices (CCD's) and complementary metal oxide semiconductor (CMOS) image sensors. Digital image sensors are incorporated within a wide variety of devices throughout the consumer, industrial and defense sectors among others.
An image sensor is a device comprising one or more radiation sensitive elements having an electrical property that changes when radiation is incident upon them, together with circuitry for converting the changed electrical property into a signal. As an example, an image sensor may comprise a photodetector that generates a charge when radiation is incident upon it. The photodetector may be designed to be sensitive to electromagnetic radiation in the range of (human) visible wavelengths, or other neighboring wavelength ranges, such as infra red or ultra violet for example. Circuitry is provided that collects and carries the charge from the radiation sensitive element for conversion to a value representing the intensity of incident radiation.
Typically, more than one radiation sensitive element may be provided in an array. The term pixel is used as a shorthand for picture element. In the context of a digital image sensor, a pixel refers to that portion of the image sensor that contributes one value representative of the radiation intensity at that point on the array. These pixel values are combined to reproduce a scene that is to be imaged by the sensor. A plurality of pixel values can be referred to collectively as image data. Pixels may be formed on and/or within a semiconductor substrate. In fact, the radiation sensitive element comprises a part of the pixel, and part of the pixel's surface area (the proportion of the pixel area that the radiation sensitive element takes up is known as the fill factor). Other parts of the pixel are taken up by metallization such as transistor gates and so on. Other image sensor components, such as readout electronics, analog to digital conversion circuitry and so on may be provided at least partially as part of each pixel, depending on the pixel architecture.
Various applications use a number of image sensors to produce a composite image. In these applications the interconnection of data between the image sensors is a significant issue, particularly in the case of pixel arrays having many thousand or many million pixels, as are increasingly commonplace.